Which drug might be responsible for a client with bipolar disorder showing a serum sodium level of 132 mEq/L?

The serum sodium level of 132 mEq/L indicates a potential case of hyponatremia, which can be associated with certain medications. Among the drugs listed, lithium is particularly known for its ability to affect electrolyte balance, including sodium levels. While fluoxetine, valproic acid, and carbamazepine do interact with neurotransmitters and can have various side effects, they are less commonly linked to significant changes in serum sodium levels compared to lithium.

Lithium therapy can lead to hyponatremia either directly through its effects on renal handling of sodium or indirectly, as patients on lithium may have altered thirst mechanisms. This can result in volume depletion or dilutional hyponatremia. Therefore, when considering the drug often associated with hyponatremia in a patient with bipolar disorder, lithium stands out as the most relevant choice.

In the context of bipolar disorder treatment, while other medications are effective for mood stabilization, none have a well-documented link to hyponatremia to the extent that lithium does. Thus, in this clinical scenario, lithium would be the drug most likely responsible for the observed serum sodium level.